Typical roller cutter journal bearings are configured to separate the operational loads of each cutter into radial and thrust components with a plurality of each type of bearing disposed within a sealed, lubricated, lug-cutter system. Radial and thrust bearings may differ by design as to their loading, but more fundamentally they differ in their contact dynamics. Sliding contact can be essentially continuous for a rotating thrust bearing, whereas any portion of a rotating radial bearing cycles in and out of contact during each revolution.
Both types of friction bearings are designed for use at very high PV ranges (pressure X velocity product), often over a million ft.lb/in.sup.2 min. Dynamic compressive operating stresses may have peak values in the 30,000-80,000 psi range, with frictional shear components superimposed. Even higher stresses may be experienced due to bending in "floating" bushing or washer elements such as disclosed in U.S. Pat. Nos. 3,721,307; 3,917,361; 3,990,751; and 4,439,050, which are favored in bit bearing designs because of their differential sliding characteristics. Moreover, ambient operating temperatures in the well bore environment are commonly between 150.degree.-400.degree. F., and seal leakage often introduces abrasive and corrosive contaminants into the bearing system at some point in the life of the bit.
These conditions substantially limit the range of suitable material couple/lubricant options available for bit use. Dissimilar metals have been used to reduce galling tendencies and extreme pressure greases with solid lubricant additives are also commonly employed. When floating members are used or when thick, continuous inlays such as disclosed in U.S. Pat. Nos. 3,995,917 and 4,416,554 are incorporated in journal or cutter surfaces, materials having minimum yield strengths of about 140,000 psi are needed to avoid macroscopic plastic deformation in service.
In addition, many material couples exhibit friction coefficients in this type of service which are sufficiently high to cause significant internal heating of the bearing system. Under such conditions, temperature may become the limiting factor in bit bearing performance due to its effects on tribological behavior and structural integrity of the bearing elements, as well as its effects on elastomer seal components commonly utilized in rotary bits. Laboratory testing has shown that transitions in the frictional behavior of the sliding surfaces are associated with thermal spikes which precede the onset of catastrophic bearing failure.
For these reasons, the ultimate load capacity, life expectancy, and performance consistency of roller cutter drill bit bearings are extremely sensitive to the mechanical and tribological behavior of bearing surfaces and substrate material. Few alternatives can provide the necessary combination of strength, ductility, temperature stability, and low friction in the operating regime of rock bits. In addition, fabricability imposes another difficult set of constraints, because of close dimensional tolerances, and the integrated, multi-element design of stationary and rotating components.
Materials used in combination with the hardened steel surfaces in bit journal bearings have included precipitation hardened copper-beryllium (shown in U.S. Pat. Nos. 3,721,307 and 3,917,361), spinodally-hardened copper-tin-nickel (shown in U.S. Pat. No. 4,641,976), aluminum bronzes (shown in U.S. Pat. No. 3,995,917), and cobalt-based stellite alloys (shown in U.S. Pat. No. 4,323,284). These materials offer suitable ambient temperature yield strengths for use as structural elements or inlays, and acceptable anti-galling properties against hardened steel. However, at elevated PVs they can undergo a transition to high-friction operation, and except for the stellites, these alloys exhibit a rapid reduction in yield strength at temperatures above about 500.degree. F. Because such surface temperatures are not uncommon in bit thrust bearings, stellites have been the structural inlay material of choice for journal surfaces. Furthermore, since heat transfer is restricted in floating elements, floating-thrust-washer designs have been largely precluded particularly for high speed bit applications. As bit journal bearings are pushed to higher PVs through the use of high speed downhole motors and increasing rig capacities, floating radial bearing elements are also becoming subject to thermal failure modes.
Discontinuous inlays or thin coatings of essentially non-structural, low-strength metals have been used to modify high-PV tribological behavior of bit friction bearings on both a micro- and macro-scale. Such materials as copper, silver, silver-manganese, lead, tin, and indium have been incorporated in bits in various ways such as described in U.S. Pat. Nos. 3,235,316; 3,990,751; 4,074,922; and 4,109,974. These discontinuous inlays or thin coatings act as surface modifiers in asperity interactions to retard galling and to serve as "solid" lubricants at pressures which exceed the film strength of greases under boundary-lubrication conditions. However, they are sensitive in their performance to placement within the system. They also reduce structural strength of the load surfaces and can add to effective bearing clearance. In addition, at elevated temperatures, the functuality of such discontinuous inlays or thin coatings may be altered as melting temperatures are approached.
Hard, thin coatings constitute another class of tribological materials which have been used with some success in rock bit bearings. These have been incorporated onto lug or cutterbore steel substrates by processes such as boronizing, ion-nitriding, and flame- or plasma-deposition of carbide composites, such as described in U.S. Pat. Nos. 4,012,238; 4,102,838; 4,618,269; and 4,969,378. Diamond surfaces have also been proposed as in U.S. Pat. No. 4,802,539. While these approaches offer low-friction and anti-galling advantages, they are limited in their use by brittleness and mechanical property mismatches with the available substrates, and by configurational constraints on their incorporation in practical bearing designs. As such, they have not been suitable for use with floating elements.
Thus, the art has not heretofore simultaneously addressed the current thermal limitations of both the surface and substrate of high-PV sliding rock bit bearings, and particularly floating bearing elements which entail higher strength and thermal stability demands.
The aerospace field has provided tremendous advances in the development of high-strength, high-temperature, corrosion-resistant "super-alloys" notably for use in turbo-machinery. These comprise nickel-, iron- and cobalt-based, solid-solution-strengthened and precipitation-strengthened alloys which exhibit superior yield strength and creep resistance at elevated temperatures. (ref: "The Superalloys" J. Wiley, New York, 1972; Metals Handbook, Ninth Ed. Volume 3, p207-268).
A parallel effort in the development of coatings to enhance the erosion- and corrosion-resistance of superalloy surfaces subjected to high-velocity gas streams has produced a variety of intermetallic coatings which are known to exhibit excellent chemical and thermal stability. Aluminides are a prominent class of such intermetallic coatings which entail ordered compounds of aluminum with other metals such as nickel, iron, cobalt, and/or titanium. In particular, conversion coating of nickel-base substrates via surface aluminimizing and high-temperature diffusion to form nickel aluminide coatings has been described in U.S. Pat. Nos. 3,544,348, 4,132,816, and 4,142,023. Minor additions of other elements such as boron, actinide- or lanthanide-series elements are also recognized as offering property enhancements of nickel aluminide materials, such as disclosed in U.S. Pat. Nos. 4,933,239 and 4,944,858. Superalloy substrates coated with aluminide materials of this type have found extensive application in turbine blades.
More recently, nickel-aluminide coatings have been observed to exhibit low-galling tendency in contact with steel in molten sodium environments. U.S. Pat. No. 4,769,210 describes such an application for plasma- or detonation-gun-coated reactor components. The galling resistance and corrosion resistance of aluminide coatings has enabled their application as a sealing surface against Hastelloy C-276 in all metal subsurface safety valves for corrosive oilfield environments.